Managing adverse effects of acid mine drainage at Stockton Opencast Coalmine : a geotechnical and geochemical approach.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Stockton Opencast Coalmine has well documented acid mine drainage (AMD), which is the result of oxidation and leaching of sulfide-rich Brunner Coal Measures waste rock. This thesis investigates the potential of mitigating AMD effects through encapsulating the waste material in a wet cover system. Specifically four locally available cap-rocks have been examined being, fresh and weathered Kaiata Formation mudstones, silt pond fines (SPF) and weathered Berlins Porphyry. These materials are tested in the laboratory, geotechnically, using grain size analysis, Atterberg limits, double-ring infiltrometer, and slake durability and geochemically, using paste pH, net acid producing potential, kinetic net acid generation, and free draining leachate columns. Field trials, using 10 m x 10 m small-scale capped waste dump pads, and computer modelling predictions, have then been applied to make recommendations on the wet cover design best suited to Stockton Coalmine. The geotechnical tests indicate the two Kaiata mudstone materials tested have the most satisfactory properties for a moisture retaining cap-rock achieving permeability >1 x 10⁻¹⁰ ms⁻¹ after compaction. The Berlins Porphyry also displays geotechnical properties suitable for capping (permeability >1 x 10⁻⁸ ms⁻¹) after compaction, but the SPF prove least favourable in geotechnical characteristics (permeability ~2 x 10⁻⁷ ms⁻¹ with the addition of cement kiln dust). The geochemical data identifies the weathered Berlins Porphyry as minimally acid producing, the SPF as potentially acid forming (PAF), and both Kaiata samples as high PAF which is a significant factor in wet cover design. The leachate data from the field trials indicated that the 1 m thick compacted Kaiata mudstone capped field trial (both fresh and weathered), and the 1 m SPF plus cement kiln dust capped field trial, performed the most efficiently in reducing acidity levels and associated metals, followed by the 1 m thick compacted Berlins Porphyry cap-rock. The O₂% profiles indicated that the compacted weathered Kaiata mudstone with the addition of topsoil slowed oxygen diffusion most effectively, followed by the 1 m compacted fresh Kaiata cap and the 1m uncompacted weathered Kaiata cap. The other tested capping regimes did not significantly affect O₂ ingress. The computer modelling showed that compacted Kaiata had adequate moisture retaining properties and also showed that both Berlins Porphyry and Kaiata mudstone cap saturation increases with time. It is recommended that Kaiata mudstone or the Berlins Porphyry is used as the 1 m thick caprock material. Where the Kaiata is identified as non-acid forming the mudstone should be selectively stockpiled for use in the wet cover system. DRI testing during after cap-rock compaction should be done at regular intervals to ensure permeabilities of <1 x 10⁻⁷ ms⁻¹ are achieved. Topsoil with a high permeability should placed on top of the cap-rock to complete the wet cover system and waste dump at approximately 20° to prevent drying out of the cap-rock. This encapsulation system should result in an improved surface water quality at Stockton Opencast.